1. Field of the Invention
This invention relates to a thermal recording method for recording a gradation image on a heat-sensitive recording medium by applying heat energy thereto by a laser beam.
2. Description of the Related Art
There has been put into wide use a thermal recording device which records an image or the like on a heat-sensitive recording medium by applying heat energy to the recording medium. Recently there has been developed a thermal recording device in which a laser is employed as a heat source, thereby making it feasible to effect high speed recording. See, for instance, Japanese Unexamined Patent Publication Nos. 50(1975)-23617, 58(1983)-94494, 62(1987)-77983 and 62(1987)-78964.
We have disclosed a heat-sensitive recording material which is used in such a thermal recording device and on which a high quality gradation image can be recorded. The heat-sensitive recording material comprises a color forming agent, a developing agent and a light absorbing dyestuff (photo-thermo conversion agent) provided on a support film and forms a color in a density according to the heat energy applied. See Japanese Unexamined Patent Publication Nos. 5(1993)-301447 and 5(1993)-24219.
The heat-sensitive recording material has a heat sensitive layer formed by applying, to a support film, coating liquid containing therein emulsion obtained by dissolving micro-capsules containing at least a basic dye precursor, a developing agent and a light absorbing dyestuff in organic solvent which is insoluble or slightly soluble in water and then emulsifying the solution.
As the basic dye precursor, is employed a compound which is generally substantially colorless, is colored by donating electrons or accepting protons of acid or the like and has a partial framework of lactone, lactam, sultone, spiro-pyran, ester, amide or the like and in which ring opening or cleavage of the partial framework occurs upon contact with a developing agent. For example, crystal violet lactone, benzoyl leuco methylene blue, malachite green lactone, rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolinonebenzospiropyran and the like can be used.
As the developing agent for these color forming agents, acidic compounds such as phenol compounds, organic acids, metal salts of organic acids, oxybenzoate esters or the like are employed. As the developing agent, those having a melting point in the range of 50 to 250.degree. C. are preferred, and phenols or organic acids which are slightly soluble in water and have a melting point in the range of 60 to 200 C. are especially preferred. The examples of the developing agent are disclosed, for instance, in Japanese Unexamined Patent Publication No. 61(1986)-291183.
As the light absorbing dyestuff, those having a low light absorption coefficient to visible light and an especially high light absorption coefficient to wavelengths in the infrared region are preferred. For example, cyanine dyestuffs, phthalocyanine dyestuffs, pyrylium and thiopyrylium dyestuffs, azulenium dyestuffs, squarylium dyestuffs, metal complex dyestuffs such as of Ni or Cr, naphthoquinone and anthraquinone dyestuffs, indophenol dyestuffs, indoanyline dyestuffs, triphenylmethane dyestuffs, triarylmethane dyestuffs, aminium and diimmonium dyestuffs and nitroso compounds can be used. Among these compounds, those having a high absorption coefficient to light in near infrared region having wavelengths of 700 to 900 nm are especially preferred in view of the fact that semiconductor lasers oscillating near infrared rays have been put into practice.
In the aforesaid recording device, two-dimensional recording of a gradation image is carried out by causing a laser beam to scan a heat-sensitive recording material in the form of a sheet by use of a polygonal mirror rotating at high speed (main scanning) while conveying the sheet in a sub-scanning direction, and converting light energy of the laser beam to heat energy by light absorbing dyestuff contained in the heat-sensitive recording material.
The heat-sensitive recording material forms a color in a density according to the heat energy applied.
Accordingly, the density of a scanning line can fluctuate under thermal influence of the scanning line recorded just before, and therefore, different from silver salt photography where there is no thermal influence, there is fear that the obtained image deviates from a desired one.
FIG. 8A shows temperature distributions a1 to a7 of main scanning lines in the sub-scanning direction where the sub-scanning frequency is 200 Hz, the diameter of the laser beam as measured in the sub-scanning direction is 120 .mu.m the recording intervals in the sub-scanning direction are 50 .mu.m and the sensitivity of the heat-sensitive recording material (.gamma. properties) is 5. FIG. 8B shows temperature distributions b1 to b7 of main scanning lines in the sub-scanning direction where the sub-scanning frequency is 900 Hz, the diameter of the laser beam as measured in the sub-scanning direction is 120 .mu.m, the recording intervals in the sub-scanning direction are 50 .mu.m, and the sensitivity of the heat-sensitive recording material (.gamma. properties) is 5. In the temperature distribution curves a1 to a7 and b1 to b7, the temperature at which the optical density becomes 1.5 is standardized as 1.0, and main scanning by the laser beam is interrupted for an interval corresponding to two main scanning lines between the temperature distribution curves a3 and a4 in FIG. 8A and between the temperature distribution curves b3 and b4 in FIG. 8B. FIGS. 9A and 9B show the density distributions corresponding to the temperature distributions of FIGS. 8A and 8B, respectively.
In the case shown in FIGS. 8A and 9A, the recording time intervals (5 ms in this case) in the sub-scanning direction determined by the sub-scanning frequency is long relative to the time constant of heat dissipation of the heat-sensitive recording material, and accordingly mutual thermal influence between the main scanning lines is very small and the temperature drop factor .DELTA.T of the temperature distribution a4 to the temperature distribution a5 is only 2% and the density drop factor .DELTA.D is only 0.1. To the contrast, in the case shown in FIGS. 8B and 9B, the recording time intervals (1 ms in this case) in the sub-scanning direction determined by the sub-scanning frequency is short relative to the time constant of heat dissipation of the heat-sensitive recording material, and accordingly mutual thermal influence between the main scanning lines is very large and the temperature drop factor .DELTA.T of the temperature distribution a4 to the temperature distribution a5 is as large as 15% and the density drop factor .DELTA.D is as large as 0.75.
Since one main scanning line is normally formed of thousands of picture elements, the main scanning frequency is much higher than the sub-scanning frequency. Accordingly, the recording time intervals for the picture elements in the main scanning direction is much shorter than the time constant of heat dissipation of the heat-sensitive recording material and thermal influence on the density of a picture element of the picture element located just before thereof in the main scanning direction is negligible.
As a result, fluctuation in density of each picture element due to mutual thermal influence of adjacent picture elements on each other appears mainly on the picture elements adjacent to each other in the sub-scanning direction of the heat-sensitive recording material. That the fluctuation in density depends upon the sub-scanning frequency of the laser beam can be understood from FIGS. 8A, 8B, 9A and 9B. The fluctuation in density also depends upon the diameter of the laser beam as measured in the sub-scanning direction and the recording intervals of the picture elements in the sub-scanning direction. Accordingly, conventionally it takes a long time to set various parameters in order to keep the density drop factor AD within a desired range.